The technical development in the area of wind turbines is leading to, i.a., larger and larger rotor diameters and tower heights. Thus, large power fluctuations due to, for example, grid faults or strong wind gusts cause a correspondingly large deflection on the tower; this in turn leads to high loads on the plant. For this reason, for example, wind turbines that for implementing a variable rotor speed generally use three-phase generators in combination with full-scale power converters are connected with large resistors via so-called choppers to the DC link of the full-scale power converter so that with a spontaneous loss of the load (for example, grid faults), the load on the rotor can be maintained and thus rapid adjustment of the rotor blades can be avoided. Rapid adjustment of the rotor blades would be necessary in a rapid load loss in order to avoid overspeed of the rotor, but it would lead to a correspondingly large change of the rotor thrust and would thus dramatically load the tower. This problem becomes greater, the higher the tower.
Similar problems can also occur in, for example, hydroelectric plants by, for example, in the case of longer-lasting grid faults, the turbine going into overspeed due to the absence of a load that under certain circumstances would cause damage to it. Likewise, for drives for industrial applications, there are also operating states in which in the case of, for example, a power failure for a short interval, a drive-side or output-side braking moment is necessary in order to bring the system into a safe state.
The time interval for recognizing the fault up to shutdown of the plant or up to the end of the grid fault can last up to several seconds, with which a correspondingly large dimensioning of the aforementioned resistors is necessary.
The method described for plants with full-scale power converters cannot be implemented, however, with classic differential systems (electromechanical, hydrostatic and hydrodynamic), since in these cases, the generator is connected directly to the grid. The same also applies, i.a., to so-called double-fed three-phase AC machines.